Typical spas are designed around dimensional lumber and are usually very linear in shape. Some deviations do occur in certain models but only on one or two sides. Spas with very linear shell shapes require very linear frames that are easily constructed with dimensional lumber, like 2×4's or 2×2 lumber.
Typical linear spas are very plain looking, especially when the cover is on and they are not in use. Linear spas are not architecturally pleasing and cannot in and of themselves become a focal point for the customers' landscape architecture. Accordingly, many spa owners hide their spas with landscaping or put them inside structures such as gazebos.
It is therefore advantageous to provide the customer with a spa shell that is curvilinear in shape so that the spa is not just a box of hot water. A spa shell that has curvilinear sides necessarily requires a frame that among other things will remain square with handling and water pressure and that will fully support the bar top. Current linear spa frame designs are inadequate for a curvilinear spa shell design. A metal frame that follows the entire profile of the curvilinear spa is complex in shape and difficult to manufacture. Additionally, such a frame would be heavy. As a result, the cost associated with such a frame would be excessive.